1. Field of the Invention
The present invention relates to an internal gear pump capable of preventing small vibrations generated in the crescent disposed between the outer rotor and the inner rotor due to pressure differences at the outlet port, so that fatigue failure of the crescent does not occur over a long period of time, and the durability is increased.
2. Description of the Related Art
Internal gear oil pumps frequently use trochoid-shaped rotors. Using trochoid-shaped gear teeth has the advantages that the inner and outer rotors are in rolling contact, so gear impact noise is small, and cavitation does not easily occur. Also, the height of the tooth (from the base to the top) can be made large, which has the advantage that the flow rate can be increased. On the other hand, however, with trochoid-shaped rotors the space between gear teeth (cells) is sealed by the line contact of the inner tooth form contacting the outer tooth form. Therefore, pressure is lost from the line contact portion to adjacent cells, there is the disadvantage that the pressure generated is not very high. Furthermore, for smooth rolling, there is a very small gap between the inner tooth form and the outer tooth form, and this is also a cause of loss of pressure. Also, there is a type of pump known as a crescent pump, in which a part known as the crescent is disposed between the inner rotor and outer rotor. In this form, there is line contact at a plurality of locations between the rotor teeth and the fixed crescent, so pressure cannot easily escape to adjacent cells. This has the advantage that a higher pressure can be generated compared with the normal internal gear pump with no crescent. Also, in conventional crescent pumps normal gears, in other words gears with comparatively low teeth, are generally used, so pressure fluctuations are not much of a problem. However, in recent years the requirements for greater efficiency and performance are increasing. In response to these requirements usually performance (flow rate) is improved by increasing the height of the teeth, and reducing the number of teeth. However, crescent pumps have the disadvantage that when the height of the teeth is increased and the number of teeth is reduced, outlet vibrations and cavitation can more easily occur.
As a result, by trying to improve the flow rate with trochoid gear teeth form, for which the gear impact noise is low, cavitation does not easily occur, and the height of the teeth can be formed larger, there is a danger of occurrence of fatigue failure of the crescent due to small vibrations generated by pressure fluctuations when the flow rate is high. Therefore, it was very difficult to achieve high performance with gear rotors with high teeth, in particular in pumps whose structure combined trochoid-shaped teeth rotors, for which the height of the teeth can be increased compared with normal gears, with a crescent. The more that performance (flow rate) was increased the more the fatigue failure problem increased. For example, as disclosed in Japanese Patent Application Laid-open No. S59-131787, with rotors whose tooth form approximated circular arcs at the top and the base of the teeth, the space between teeth (cells) in the parts approximated by circular arcs is narrower than the space between teeth with normal trochoid curves. Therefore the quantity of oil that can be delivered in one revolution is reduced, and the performance (flow rate) is reduced accordingly. However, the resulting pressure fluctuations are small, so conventionally this was not such a big problem, but high efficiency, high performance pumps using trochoid rotors is a big problem.
Furthermore, in that internal gear pump, there is a slight time difference between the timing that the cells formed between the inner rotor and the crescent and the cells formed between the outer rotor and the crescent link with the outlet port. In other words, in an internal gear pump, the rotation speeds of the outer rotor and the inner rotor are different. The rotation speed of the inner rotor is faster than the rotation speed of the outer rotor. Therefore, normally the time that the top of a tooth of the inner rotor separates from the crescent does not coincide with the time that the top of a tooth of the outer rotor separates from the crescent. Therefore, the time that the fluid in a cell on the outside of the crescent flows into the outlet port does not coincide with the time that the fluid in a cell on the inside of the crescent flows into the outlet port. Because one of these cells is first to link with the outlet port, a pressure difference arises between the cell on the inside of the crescent and the cell on the outside of the crescent.
This pressure difference causes small vibrations to occur in the crescent. These small vibrations could cause fatigue failure in the crescent. This phenomenon arises regardless of the shape of the teeth. It is considered that with trochoid-shaped teeth the extent of the outlet vibrations is small, but in internal gear pumps that use a crescent this problem can easily arise.